1. Field of the Invention
This invention relates to water purification by reverse osmosis.
2. Prior Art
Reverse osmosis is a process in which pressure is applied to a solution to force water from a solution through a semipermeable membrane so leaving a more concentrated solution to the inlet side. The pressure required has to be sufficient to overcome the osmotic pressure and hence becomes greater as the solution is more concentrated. For brackish water, a pressure of the order of 300-500 psi (2000-3500 k Nm.sup.-2) whilst for sea-water, since the osmotic pressure is higher, a pressure typically of 600-1000 psi (4000-7000 k Nm.sup.-2) might be necessary. One of the main problems restricting commercial use of this process is the high power consumption and hence high cost.
Apparatus for the reverse osmosis purification of water is described for example in U.S. Pat. Nos. 3,825,122; 3,449,245; 3,355,382; 3,637,081; 3,654,148; 3,707,231; 3,228,876 and 3,405,058. As is disclosed in these specifications, it has been appreciated that, with the high pressures involved, there is considerable waste of power unless use can be made of the pressure energy in the high pressure fluid which passes over the membrane, i.e. the unpurified water which has eventually to be discharged to waste. For example U.S. Pat. No. 3,405,058 makes use of a fluid motor driven by this high pressure fluid, which motor drives a pump used in the water purification system. U.S. Pat. No. 3,875,122 makes use of the high pressure fluid to act on a piston which in turn drives a piston used as a pump in the water purification system; the use of the very high pressures acting on the pistons raise sealing problems and such an arrangement requires synchronisation of the opening and closing of a large number of valves.